For microwave amplifiers used in communication-related applications, there are instances in which a plurality of communication carriers having different frequencies in a fixed operating band is input simultaneously. However, for microwave amplifiers, mutual interaction may occur between a plurality of communication carriers due to the generation of a beat signal between the plurality of communication carriers with different frequencies.
Patent Literature 1, for example, proposes a microwave amplifier which has a good distortion characteristic even when many communication carrier frequencies are included in a microwave signal configured to be amplified. For the microwave amplifier in Patent Literature 1, a filter circuit which becomes a high impedance with the communication carrier frequency of a microwave signal and becomes a low impedance with the frequency of a beat signal that is generated from a plurality of communication carrier frequencies is connected to a drain of a field effect transistor (FET). Further, a capacitor which short-circuits a beat signal is connected between the other end and a ground (GND).
Also, Patent Literature 2, discloses about reducing significantly the gain in low frequency bands which causes a problem when amplifying a plurality of communication carriers of a microwave amplifier, and reducing low frequency components on the input and output side of an amplifier. In the microwave amplifier of Patent Literature 2, ends of bias supply lines having predetermined lengths are connected to a connecting point of a microwave amplifier circuit having a semiconductor element and an input side transmission line, and to a connecting point of the microwave amplifier circuit and an output side transmission line respectively. Capacitors are connected to other ends of the bias supply lines, respectively. A parallel circuit consisting of a resistor and a capacitor is loaded between the connecting point of the input side bias circuit to the input side transmission line and the input terminal of the microwave amplifier circuit.
It is known that, in microwave amplifiers, a resistor is added to a bias supply circuit in order to reduce ultra-high frequency power losses and stabilize operation of semiconductor elements at low frequencies. In Patent Literature 3, for example, the ultra-high frequency semiconductor circuit is configured to be provided with a capacitor in parallel with a resistor in a bias circuit, the capacitor having a sufficiently small reactance at an ultra-high frequency in the vicinity of the necessary frequency and having a large reactance at a low frequency.